Floppy disc subsystems have generally used lead screws which extend from near the center of the floppy disc to a stepping motor which is located at a radial position beyond the outer periphery of the disc. Normally the stepping motor is rigidly secured to a baseplate as is the disc drive. Further, the lead screw is normally integral with the drive shaft of the stepping motor, and the inner end is often merely journalled in a bearing adjacent the center of the disc. Accordingly, as the floppy disc unit is set into operation and used for extended periods of time, the baseplate, the stepping motor, and the lead screw, as well as the floppy disc may all heat up to various final temperatures, and may also have many combinations of intermediate thermal variations. These factors mean that the position of the head, which is determined by the lead screw, can shift to a very significant extent relative to the radial position on the disc. In addition, successive steps in a stepping motor may often be shifted significantly as compared with the expected angular increment of the stepper motor. These discrepancies in the steps result from mechanical variations in manufacturing and the like. In addition, the friction forces in the stepping motor introduce a significant residual error in angular position for the stepper motor. And finally, additional errors in track location may be introduced by the loose fit of the magnetic disc on the driving hub. As a result of these adverse factors, most floppy disc subsystems only have approximately 48 tracks per inch, with a resultant track spacing of about 21 thousandths of an inch.
Reference is also made to S. A. Morgan U.S. Pat. No. 4,001,888, granted Jan. 4, 1977, and P. Lesca, U.S. Pat. No. 4,024,581, granted May 17, 1977. These and other patents show arrangements which would improve the location of the magnetic head relative to the magnetic disc; however, the designers apparently did not appreciate the fact that a complete analysis and design is required to avoid loss of information, and that improving the tolerances in one part of the design is of little or no significance, if loose tolerances are present elsewhere.
A principal object of the present invention is to significantly reduce the variable factors outlined above, and thus greatly increase the accuracy of head positioning and/or data storage capacity of the floppy disc apparatus.